a ring, or a ring on a pivot, the ring being formed partly of bone, partly of ligament. In the proximal radioulnar articulation, the ring is formed by the radial notch of the ulna and the annular ligament; here, the head of the radius rotates within the ring. In the articulation of the odontoid process of the axis with the atlas the ring is formed in front by the anterior arch, and behind by the transverse ligament of the atlas; here, the ring rotates around the odontoid process.

Condyloid Articulation (articulatio ellipsoidea).In this form of joint, an ovoid articular surface, or condyle, is received into an elliptical cavity in such a manner as to permit of flexion, extension, adduction, abduction, and circumduction, but no axial rotation. The wrist-joint is an example of this form of articulation.

Articulation by Reciprocal Reception (articulatio sellaris; saddle-joint).In this variety the opposing surfaces are reciprocally concavo-convex. The movements are the same as in the preceding form; that is to say, flexion, extension, adduction, abduction, and circumduction are allowed; but no axial rotation. The best example of this form is the carpometacarpal joint of the thumb.

Enarthrosis (ball-and-socket joints).Enarthrosis is a joint in which the distal bone is capable of motion around an indefinite number of axes, which have one common center. It is formed by the reception of a globular head into a cup-like cavity, hence the name ball-and-socket. Examples of this form of articulation are found in the hip and shoulder.

Arthrodia (gliding joints) is a joint which admits of only gliding movement; it is formed by the apposition of plane surfaces, or one slightly concave, the other slightly convex, the amount of motion between them being limited by the ligaments or osseous processes surrounding the articulation. It is the form present in the joints between the articular processes of the vertebræ, the carpal joints, except that of the capitate with the navicular and lunate, and the tarsal joints with the exception of that between the talus and the navicular.

4. The Kind of Movement Admitted in Joints

The movements admissible in joints may be divided into four kinds: gliding and angular movements, circumduction, and rotation. These movements are often, however, more or less combined in the various joints, so as to produce an infinite variety, and it is seldom that only one kind of motion is found in any particular joint.

Gliding Movement.Gliding movement is the simplest kind of motion that can take place in a joint, one surface gliding or moving over another without any angular or rotatory movement. It is common to all movable joints; but in some, as in most of the articulations of the carpus and tarsus, it is the only motion permitted. This movement is not confined to plane surfaces, but may exist between any two contiguous surfaces, of whatever form.

Angular Movement.Angular movement occurs only between the long bones, and by it the angle between the two bones is increased or diminished. It may take place: (1) forward and backward, constituting flexion and extension; or (2) toward and from the median plane of the body, or, in the case of the fingers or toes, from the middle line of the hand or foot, constituting adduction and abduction. The strictly ginglymoid or hinge-joints admit of flexion and extension only. Abduction and adduction, combined with flexion and extension, are met with in the more movable joints; as in the hip, the shoulder, the wrist, and the carpometacarpal joint of the thumb.

Circumduction.Circumduction is that form of motion which takes place between the head of a bone and its articular cavity, when the bone is made to circumscribe a conical space; the base of the cone is described by the distal end of the bone,